By David Seland, Principal and Founder of ISE Logik
When a new concrete slab is placed, curing is a critical process for achieving the overall strength, durability, performance and in many aspects, aesthetics everyone on a project expects of concrete. How that concrete is cured, and more specifically what products may be used for curing, can have a direct impact on the successful installation of the floorcovering system.
Curing products with silicon dioxide from a silicate ingredient are commonly used and penetrate the surface of the concrete. This becomes a permanent component of the concrete. Only with aggressive shot blasting of the concrete’s surface can this be removed, as these products reach depths into the concrete up to 1/4” and more. Surprisingly (or maybe not) floor patch, levelers and floorcovering adhesives are routinely applied to these surfaces successfully, without removal or isolation of the concrete curing product.
Proper curing of concrete reduces cracking, improves durability and increases strength. Conversely, when concrete is improperly cured, it can lead to inadequate strength gain, surface dusting, surface crazing, plastic shrinkage cracks, excessive slab curl, reduced resistance to freeze/thaw, scaling and on and on.
Curing conditions
Curing relies on two conditions: suitable moisture and temperature. This is because water is a chemical necessity so that hydration and pozzolanic reactions can occur; these reactions are what turns the wet cementitious material and aggregate mix into a hardened mass.
As soon as that wet mix leaves the ready-mix concrete truck, ambient conditions immediately begin to take some of that water away through evaporation. Elevated ambient air temperature, low ambient relative humidity, wind, high concrete mix temperature (from heat of hydration, hot aggregate that has been in the sun and high temperature for extended periods, etc.) and any combination of these all work to rapidly remove necessary water from the mix.
This evaporation can potentially occur at any ambient temperature; the need to protect against excess evaporation is not solely related to warm weather conditions. Curing is not just about keeping sufficient moisture in the freshly placed concrete—it is also about maintaining a suitable temperature for and within the concrete. So ambient temperature, mix temperature and the temperature of the base on which the concrete is to be placed all must be discussed, monitored, planned for and addressed.
In addition to specific curing methods and curing products being applied to the concrete, the concrete finisher may utilize an evaporation retarder on the surface of the concrete. These evaporation retarders are designed to form a thin film on the concrete surface, reducing the rapid moisture loss of the concrete prior to curing. On a hot summer day, evaporation retarders are commonly used so the concrete surface condition is normalized and the contractor can maintain the finishing schedule for the concrete. A significant portion of the evaporation retarder may be water, but other ingredients such as emulsified cetyl alcohol, stearic acid, sodium silicate, potassium silicate, nano-silicate, etc. are frequently ingredients in these products.
Methods of curing
In ACI 308, the American Concrete Institute discusses a few general systems for maintaining adequate moisture content so that freshly placed concrete can achieve the desired design properties:
- The continuous or frequent application of water through ponding, fogging, steam or saturated cover materials such as burlap or cotton mats, rugs, sand, and straw or hay; none of which are recommended for slabs to receive a moisture-sensitive covering.
- The minimization of water loss from the concrete by use of plastic sheets or other moisture-retaining materials, placed over the exposed surfaces for three to seven days after concrete placement which is highly impractical on most jobsites, especially with elevated slabs.
- By the application of a membrane-forming compound (commonly referred to as a curing compound) meeting the requirements of ASTM C309 or C1315.
For concrete slabs to receive moisture-sensitive flooring or coatings, most of the above methods are completely impractical, unfeasible, too costly or time-consuming. Further, the duration of curing required to achieve the desired levels of strength, durability or both, depends on a complex set of factors that makes it difficult to confidently state the minimum curing time. Therefore, since most of the recommended curing methods simply cannot be used with slabs (given the cost, unknown duration, wind, disruption to other trades, overall project timelines and so on), in most cases concrete slabs are cured with curing compounds. Ask any concrete contractor for commercial projects what the go-to method is for curing, and the answer will be curing compounds.
Curing compounds
As with the term curing, there exists some substantial misconceptions surrounding curing compounds; namely, many believe they are all the same. From this misconception, some design and project teams have been influenced to not specify nor use these products. The truth about curing compounds, however, is they are not all the same.
Curing compounds can be separated into two major classifications: those that are only for curing freshly placed concrete, and those that are formulated to cure and seal freshly placed concrete. To aid in better understanding these products, there are two ASTMs that cover each classification:
- ASTM C309, “Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete”
- ASTM C1315, “Standard Specification for Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete”
When laid side by side, there are enough similar paragraphs and properties across both ASTM documents that if one did not read carefully, it could be easy to misinterpret that the products are relatively equal. As an example, both ASTM C309 and ASTM C1315 describe “liquid membrane-forming curing compounds” that will not negatively react with the concrete surface, and both have water loss properties and drying time requirements. However, ASTM C1315 products have a much more stringent water loss requirement than ASTM C309 products, and ASTM C1315 requires a pull off test for adhesive applied over the cure and seal curing compound using ASTM D1734 with a minimum of 70 psi.
Most floorcovering adhesive applications have to follow very specific instructions for a porous or non-porous substrate surface, and the latest version of ASTM F710 requires all concrete slab substrate surfaces be tested for porosity either following the manufacturer’s written instructions or ASTM F3191. Why mention this? Because an ASTM C1315 cure and seal product will result in a non-porous substrate surface in almost all cases where the slab has also been finished by power troweling.
Proper curing means suitable temperature and moisture conditions “within” the freshly placed concrete, and there are few things that can damage freshly placed concrete more than improper curing. As previously mentioned, most all curing methods are simply impractical, too expensive, or too disruptive to other project trades and timelines to be properly carried out on slabs that will receive floorcoverings.
One should assume that an ASTM C1315 or ASTM C309 compliant product, and in many cases an evaporation retarder, have been used on the concrete. Bond tests with all of the flooring system products that will be applied to that concrete surface and testing the substrate surface for porosity must be performed, and choosing the correct installation products that are compatible with those test results are critical. The success of the floorcovering installation depends on it.
